We received this great info from Dr. Michael “Shane” McLellan – CEA, Ag at the Texas A&M AgriLife Extension Service, McLennan County in Waco. If you have further questions, you can reach him at (254)757-5180 or firstname.lastname@example.org.
“We have been dry since early summer with little to no rainfall over most of Central Texas. I have been receiving a lot of phone calls from concerned livestock producers in regards to prussic acid and nitrate poisoning.
Prussic acid primarily occurs in sorghum type forages less than 1.5 foot tall. I am talking about johnsongrass, grain sorghum, sorghum-sudan type forages growing in our pastures. Prussic acid does not occur in pearl millet or corn. Poisoning actually occurs from consumption of plant parts with high levels of prussic acid. Prussic acid can be associated with fast growing plant tissue. You will normally see prussic acid poisoning in young plant tissue that is growing on a plant that has been stressed or damaged. After a hard frost, a heavy N fertilizer application, grass growing in a plowed field, or drought stressed forage that received enough moisture to trigger growth. Symptoms from animals that have consumed a high enough level of prussic acid will show labored breathing if detected early, and can occur 10-15 minutes after consumption. Prevention is the best thing and it is recommended you defer grazing injured plants until they recover. After a hard freeze, severe drought, avoid grazing. According to Extension forage specialist producers should wait at least 2 weeks after plants begin growing following a rain or irrigation on drought stressed pastures. Sorghum sudans that contain prussic acid can be cut for hay and fed with little to no side effects if allowed to cure properly. You can monitor prussic acid levels by taking samples and having them tested at the Texas Veterinary Diagnostic Medical Lab.
Nitrate poisoning of livestock can occur from animals grazing sorghums, millets, corn, oats, wheat, ryegrass, and pigweed. This disorder occurs after drought or cloudy conditions prevent normal plant growth. Under this condition, the plant accumulates nitrates primarily in the stems and lower leaves instead of converting the nitrate to protein. Toxic levels of consumed forages usually exceed 1%. The term nitrate toxicity is commonly used but the toxic principle is actually nitrite. Nitrate is converted to nitrite in the rumen. Nitrite is absorbed from the rumen converting blood hemoglobin to methemoglobin. Methemogoblin cannot transport oxygen to body tissues, so animals actually die from oxygen insufficiency. You will see labored breathing in animals if you detect it early enough. To prevent, do not graze during stress periods, monitor nitrate (NO3) levels to determine levels in forage are safe, don´t graze too short nitrates accumulates mainly in stems and older leaves, don´t feed high nitrate forage free choice. Nitrate does not dissipate from hay like HCN (prussic acid). Once high nitrates levels are reached they stay high. (it must be diluted by feeding it mix with hay that is nitrate free and or discarded). Horses and hogs are less tolerant than ruminants. Plant samples can be sent to:
- Texas Veterinary Diagnostic Medical Laboratory (TVMDL)
- Phone: (979)845.3414 or 1.888.646.5623 (same day results). Fax: (979)845.1794
- Texas Cooperative Extension, Soil, Water, and Forage Testing Laboratory.
- Phone: (979)845.4816 (24 hour results)”